Limit switch



Nov. 14, 1950 A. A. DICKE 2,529,652

LIMIT SWITCH Filed Nov. 16, 1948 2 Sheets-Sheet l INVEN TOR. ALLEN/4.D/CKE j mrm A T TOIQNEYS Patented Nov. 14, 1950 LIlVIIT SWITCH Allen A.Dicke, Montclair, N. J assignor to Crown Controls Company, Inc., NewBremen, Ohio, a

corporation of Ohio Application November 16, 1948, Serial No. 60,210

6 Claims.

This invention relates to an improved limit switch and particularly tothermostatically controlled electrical switches and more particularly toa duplex apparatus for automatically opening and closing differentcircuits alternately.

An object of this invention is to provide a switch of this typeproviding for rapid or snapaction. Another object is to provide a switchof this type comprising a flexible arm carried by an abutment and havinga free end cooperating with a contact, the arm being formed between itsends with convolutio'ns to increase the resiliency thereof withoutincreasing the distance from the abutment to the contact.

While the present invention is applicable to a wide range of uses, forillustrative purposes, but without intent to unduly limit theapplication or scope of the invention, it is herein shown and describedas an automatic heating furnace control for either hold fire or highlimit operation for which. it is especially desirable, but obviously isnot limited thereto. The present instrument may be so mounted as to beresponsive to temperature fluctuations of a furnace jacket bonnet or ofa flue pipe or stack and serve to energize damper actuating means orfuel feeding mechanism as necessitated by heating requirements.

The present invention is a further development of an improvement on thatdisclosed in appli- 30 cation for Letters Patent of Wilhelm C. Muth,

Serial Number 683,054, filed July 12, 1946, now Patent No. 2,516,525,granted July 25, 1950. Said Muth application contemplates a limit switchof similar construction providing for snap-action contact operation bymeans of permanent magnets acting upon the contact arms. Due to thelimited space available said prior construction utilized magnets whichtend to pull the contact arms in the same direction as that toward whichthey are impelled by the thermo-responsive mechanism. This was found tobe unsatisfactory under certain conditions and it is an object of thepresent invention .to overcome said difficulties and to provide amagnetically restrained snap-action objects in view as will more fullyapear in the specification, the invention intended to be protected byLetters Patent consists of the features of construction, the parts andcombinations thereof, and the mode of operation, or their equivalents,as hereinafter described or illustrated in the accompanying drawings.

In the accompanying drawings, wherein is shown the preferred, butobviously not necessarily the only form of embodiment of the invention:

Fig. 1 is a front view of an assembled thermostatically energizedelectric limit switch illustrating the present invention, the coverhaving been removed.

Fig. 2 is a vertical, longitudinal cross-sectional view taken on theline 22 of Fig. 1.

Fig. 3 is a fragmentary cross-sectional View taken on the line 33 ofFig. 1.

Fig. 4 is taken along line 44 of Fig. 1.

Fig. 5 is a fragmentary front view of part of the mechanism shown inFig. l, but on an enlarged scale showing in dotted lines the position ofthe switch blade when in normal unactuated position and showing in fullline the switch blade when partly actuated and just prior to itsmovement from one contact to the other.

Fig. 6 is a view similar to Fig. 5 with the parts shown in the positionswhich they occupy after the switch blade has been actuated, and Fig. 7is a diagrammatic view of a system of electrical circuits under controlof a limit switch of the type shown in Figs. 1 to 6.

Like parts are indicated by similar characters of reference throughoutthe several views.

In the drawings, I indicates a helically wound bimetal thermallyresponsive element through which extends a rock shaft 2, to theextremity of which one end of the bimetal element l is attached at 3.

The opposite endof the thermally responsive spring I is attached to arelatively fixed sleeve 4 through which the shaft 2 axially extends.This thermally responsive portion of the apparatus may be enclosed in atubular. heat conductive limit switch in which a permanent magnet is sorelated to the contact blade as to temporarily restrain the contactblade against the action of the thermo-responsive actuator.

Another object is to provide such a switch in which the contacts are soarranged that the gap between contacts which are about to be closed isfirst gradually increased and is thereupon rapidly closed, thusincreasing the velocity of 010- sure.

housing (not shown in drawings) closed at its extreme end and has at itsopposite end a lock collar l surrounding the sleeve 4 and engageablewith a mounting plate 8, which may be attached to a portion of a furnacejacket bonnet wall or a portion of a flue or stack pipe.

Mounted. on the forward end of the sleeve 4 is a housing 9 enclosing thecontact members, their mounts and the actuating head, and having thereina bearing 2o for the shaft 2, mounted on With the above primary andother incidental Qt! abridge 9a. within the housing 9. Carried on theflanged face of the housing 9 are two contact mounting segments. Onesuch mounting segment l i is normally fixed, but is adjustable withinthe range of segmental slots i2 through which engage clamp screws 94.The other mounting segment i5 is pivotally mounted concentrically withthe housing 9 preferably on the rock shaft bearing for manualoscillatory adjustment thereabout. On each of the mounting segments Hand It a bracket It is mounted for oscillatory adjustment about a pivotil within the range of an arcuate slot l8 through which a clamp screw l9extends. Each of the brackets 16 carries a flexible spring contact armor. blade. The spring arm or blade 28 of the stationary mounting segmentl i is alternately engageable with each of a pair of relatively spacedcontacts 2-! and 22. The corresponding adjustable mounting segment i5carries a spring contact arm 23 alternately engageable with spacedcontacts 24 and 25. The spring arms 20 and 23 are normally biased-towardeach other by adjustment of the brackets it, whereby they tend to engagethe innermost contacts 2i and 24 of the respective pairs thereof, underthe inherent tension of the spring arms 26 and 23. Fixedly attached tothe end of the rock shaft or stem 2, for unison motion intermediate thespring arms 28, and 23 is an oscillatory actuating member or cam 26. Thecam 25 is rotated to and fro as the bimetal thermally responsive elementis actuated in accordance with fluctuations of temperature. The cam 26may engage one or the other of the spring arms, thereby overcoming theinherent bias of the engaged spring arm and breaking its contact withthe inner or normally engaged'contact 2! or 24 causing it'to engage theother contact of the particular pair thereof.

In order to give to the spring arms actuated by the oscillatory cam asnap-action and definition to their movement, permanent magnets 2i and24 are mounted on the corresponding mounting sectors adjacent to thecontacts 2! and 2 2 respectively, which accelerates its movement andinsures firm contact engagement.

A detachable cover cap'23'engageable withthe housing 9, encloses thespring arms, contacts and oscillatory sector. An arm 29' upon theadjustable mounting segment l5 extendsbeyond the periphery of thehousing 9 and cover cap 28, to enable the segment and with it the springarm 23'and coacting contacts 24 and 25, to be shifted circumferentially,thereby advancing or delaying the engagement of the cam 26 with the arm23 and movement of the latter away from contact 24 into engagement withcontact-25. When under increased temperature, the stress of the bimetalelement increases, and, the shaft or stem 2 is rocked to move cam 26 outof its engagement with the spring arm 20 the latter disengages thecontact 22 and, by itsinherent tension returns to engagement with theopposite contact 2|. By continued rotation the cam 26 engages the0pposite spring arm 23 to break its contact with the contact 2d and flexthe arm into engagement with the companion contact 25. The intervalbetween the release of the spring arm 28 and engagement of the springarm 23 is dependent upon the degree of rotative setting of theadjustable mounting segments II and i5. The adjusting arm 29 cooperateswith a graduated scale upon a lip or segment 80 projecting from thehousing 9 having, thereon a range of graduations from Mild through"lflormal to Cold. The arrangement is such that a lower temperature isrequired to cause the cam 26 to engage the spring arm 23 when themounting segment 55 is at one limit of its range of adjustment (Mild)than when it is oscillated to the opposite limit of its range ofmovement (Cold).

A connection panel 3| surmounts the housing 9 having thereon a series ofcircuit terminals, the conductors from which extend through the wall ofthe housing 9 to the various contacts.

Because of excessive range of temperature tending to actuate the cam 26to an extreme position no positive stop is provided, but after havinginitially actuated the engaged spring arm, the cam 26 is capable offurther idle operation in which the cam periphery merely wipes upon thespring arm under continuing influence of the thermostatic spring lwithout furtherv moving the engaged spring arm and therefore withoutstraining or breakage of parts, and without causing the springs, eithercontact or thermostatic, to take a definite set.

In operation, the switching unit functions as a hold-fire or high-limitcontrol, depending upon the condition of the fire as is reflected by thebonnet 0r stack temperatures. If thefire'is within normal limits, thecam 26 does not contact either spring arm 20 or 23. These arms thereforeremain in their normal or biased positions, in engagement with thecontacts 2i and H respectively. In this position, the switching unitperforms merely a coupling function, serving as a terminal board throughwhich the furnace control 32 is operatively connected with the-r0omthermostat 53. So long as the switching unit remains in its normalposition, operation of the furnace control unit 32 is under the solecontrol of the room thermostat, and responds to a call for heat by theroom thermostat increases combustion in the furnace, or by reducingcombus tion in the furnace when the room' thermostat is satisfied.

Power for the furnace control 32 is supplied by the secondary of atransformer 34- through the wires 35 and 33. The room thermostat and thelimit switch comprising the subject matter of the present invention areconnected to the furnace control by means of the terminals 37, 38 and39-. Terminal 33 is the off or stop terminal while the terminal 39controls the on or start operation of the control 32. The twocircuitsarecompleted through the common terminal.

As shown in Fig. 7 the system is in its off or stop position. Should theroom thermostat 33 call for heat, the arm will move from the off or stopcontact M3 to the on or start contact M. This movement of the roomthermostat moves the furnace control'unit 32- to its on or startposition, it being energized by'the following circuit; from the commonterminal 31 conductor 52 leads to terminal lr' on the panel 35 of theswitching unit. From there, through the internal wiring f the limitswitch, the current is carried through conductor 44 to the spring arm 23and thence through contact 24 and conductor 45 to the terminal it onthepanel 3i. From terminal 45 conductor 4i leads to the room thermostat 33.The circuit is completed from the thermostat through contact 4!,conductor 8 to the terminal isonthe panel-3l, and thence throughconductor 5% to terminal 39 of the furnace control 32,

The furnace control remains in its on or start position until the roomthermostat 33 is satisfied, at whichtime, the'thermostat moves fromcontact li to contact 53 thereb moving the furnace control to its off orstop position. The circuit for the off movement of the furnace.;.control is the same as for the on movement from thecommon terminal 31 ofthe furnace control to the thermostat 33. From the thermostat however,the circuit is completed through contact and conductor 5| to terminal 52on the panel 3|. From terminal 52, the internal wiring of the switchingunit carries the current through conductor 53 to the spring arm 20,thence through contact 2|, conductor 54, conductor 55 to terminal 56 onthe panel 3|. Conductor 57 interconnects terminal 56 with the foff orstop terminal 38 on the furnace control thereby completing the circuitto move the furnace control to its oiff position.

Should an abnormal fire condition arise at any time, such condition isrectified by automatic operation of the limit switch of the presentinvention. For example, if during the on period of operation of ;thefurnace control, as called for by the room thermostat 33 the flue orbonnet temperature should become dangerously high, the present switchingunit responds to such abnormal condition by energizing the off or stopcircuit f the furnace control. An increase in flue or bonnet temperaturecauses clockwise rotation of the cam 26 thereby moving the spring arm 23away from contact 24 and into engagement with contact 25. Such movementof the arm 23 opens the circuit to the room thermostat therebyeliminating the overall control by the room thermostat, and substitutesa circuit to the off or stop terminal 38 of the furnace control. The newcircuit is the same as before described from the common terminal 31 ofthe furnace control to the spring arm 23.

From the arm 23 however, the circuit is completed through contact 25,conductor 55, terminal 56 on the panel 3| and conductor 51 to the off orstop terminal 38 of the furnace control. The furnace is therefore shutoff because of the increase in the flue or bonnet temperature above apredetermined safe maximum.

Another abnormal condition which is cared for and controlled by thepresent switching unit, is a decrease in flue or bonnet temperature whenthe room thermostat does not call for heat during extended periods.These periods usually occur in the early fall and late spring, whenbecause of the ambient temperature, the thermostat remains satisfied forlong periods of time. During these. periods it is quite possible for thefire to become quite low, or extinguished because of the lack ofperiodic firing operations. The low fire is reflected by a decrease inthe stack or bonnet temperature, and when such temperature falls below apredetermined minimum, the cam 26 of the present switching unit bmovement in a counter-clockwise direction, movesthe spring arm 23 fromcontact 2| into engagement with contact 22. Such movement ofthe arm 20sets upa new circuit to energize the furnace control so as to move it toits on or start position. Such on operation of the furnace control isentirely independent of any demand by the room thermostat 33.

Referring again to Fig. '7 in which the system is shown in its off orstop position with the room thermostat fully satisfied as shown by itsengagement with contact 40, upon movement of the arm 26 into engagementwith contact 22, the on circuit is completed as follows. From the commonterminal 31 of the furnace control the circuit to the thermostat 33 isthe same as heretofore explained. The circuit to arm 20 is throughcontact 40, conductor 5|, terminal 52 on the panel 3| and conductor 53,The arm 20 being in en a ement with contact 22, conductor 58, terminal49 on the panel 3|, and conductor 56. Thus the furnace control 32 isenergized so as to move to the on or start position in-- dependently ofthe room thermostat 33.

Should the room thermostat 33 call for heat during the on operation ofthe furnace control under control of the arm 20, the thermostat movesfrom contact 40 to contact 4|, thereby establishing the normal oncircuit which is completely independent of the on circuit controlled bythe arm 2|]. Upon moving ofthethermostat from the contact 40 intoengagement with the contact 4|, the special control circuit through thearm 20 is broken thereb relieving the switch 20-22 of all controlfunctions, even though the arm 20 remains engaged with the on contact22.

The contact surface of the cam or actuator 26 is concentric with itsaxis of rotation whereby it makes only point engagement with one or theother of the straight fiat circuit closing arms. The construction issuch that after having initially engaged and moved the circuit closingarm, the actuator may continue torotate to greater or less degree,without varying the adjustment of the circuit closer arm.

Referring to Fig. 3, it will be noted that coritact 24 is in the form ofa rivet passing through a ring, shaped permanent magnet 24' and througha hole in the L-shaped bracket 24 which bracket is mounted on thesegment I 5 by means of a rivet 24a and preferably also" passing througha terminal clip 24b to which is connected the conductor 45. The contactblade 23 has riveted to it a washer 240 by means of a contact rivet 24d.The washer is made of soft iron or steel and acts as an armature inrelation to the permanent magnet 24'.

Contact 25 is preferably in the form of a rivet carried on a bracket 25which may be identical with the bracket 24" and is held in place b arivet 25a to which is connected, underneath segment IS, the conductor55. It will be noted that the brackets 24" and 25 are reversed inposition. This is desirable so that the housing 9 may be made smaller indiameter as it is essential that the' rivet 25a be located within thehousing 9.

The snap-action operation of the high-limit control switch is moreclearly shown in Figs. 5 and 6. Referring to Fig. 5, the dotted linesindicate the position and form of contact arm 23 when not acted upon bycam 26. In full lines is shown the position and form of arm 23 whenacted upon by cam 26 but prior to the movement of arm 23 to breakcontact 24. It will be noted that the attractive force of magnet 24 onits armature 240 is sufficient to cause considerable distortion orflexing of arm 23, considerable energy being stored therein. As soon asthe distortion of arm 23 becomes great enough to overcome the power ofmagnet 24 contact 24 1s opened and the blade 23 moves rapidly to closecontact 25. This is done by utilizing the energy stored in spring 23. Byforming spring 23 with S shaped convolutions such as shown, theflexibility is substantially increased without increasing the distancefrom the point of support of spring 23 on bracket Hi to the contacts.Therefor it is possible to get the desired action in an instru ment ofmuch smaller dimensions than if a straight arm 23 were used as in theMuth application referred to, It will also be noted that during thefirst action of cam 26 on arm 23 the gap of contact/ 25 is increased.This is most desirable in that the subsequent closing movement ofcontact 25 is therefore much more rapid. This action is made possible bylocating the contacts 24-25 at different distances from the supportedend of arm 23. Since during the initial stressing of arm 23 the contact24 acts as a point of oscillation, the elements of contact 25 areseparated, thus-offsetting any danger of having both contacts closedduring that time.

The'operation of the low limit switch comprising arm 2i] and contact 2l22 is the same as that just described with reference to the high limitswitch except that automatic motion occurs as a result of contourclockwis movement of cam 26.

It will be noted that the constructions shown and described will serveadmirably to accomplish the objects stated above. It is to beunderstood, however, that .the constructions disclosed above areintended merely as illustrative of the invention and not as limiting asvarious modifications therein may be made without departing from theinvention as defined by a proper interpretation of the claims whichfollow.

I claim:

1. A limit switch for use in temperature control systems comprising, incombination, a thermo-responsive actuator, a mounting member, asupporting bracket on said mounting member, a flexible contact bladecarried by said supporting bracket in a position where it may be engagedby said actuator, and at least one contact carried by said mountingmember in a position where it is engageable by said contact blade, amagnet mounted adjacent said contact, and magnetic material associatedwith said contact arm so that the contact arm is attracted toward saidcontact, the contact arm being formed with convolutions between thepoint where it may be engaged by the actuator and the point where it isengageable with the contact, whereby the resiliency of the contact bladeis increased so that energy may be stored therein by the actuator whilethe magnet is holding the contact blade in engagement with the contact,until the convoluted contact blade is strained sufficiently to overcomethe pull of the magnet whereupon the contacts will be separated quicklyby the energy stored in said convoluted contact blade.

2. A limit switch for use in temperature control systems comprising, incombination, a thermo-responsive actuator, a mounting member, asupporting bracket on said mounting member, a flexible con-tact blade.carried by saidsupporting bracket in a position where it may be engagedby said actuator, and at least one contact carried-by said mountingmember. in aposition where it is engageable by said contact blade, amagnet mounted adjacent said contact, magnetic material associated withsaid contact arm so that the contact arm is attracted toward saidcontact, and a second contact on the other side of said contact arm andlocated further from said supporting bracket than said first contact,the contactarm being formed with convolutions between the point where itmay be engaged by the actuator and the point where it is engageable withthe first contact, whereby the resiliency of the contact blade isincreasedso that energy may be stored therein by the actuator while themagnet is holding the contact blade in engagement with the contact,until the. convoluted contact blade is strained sufficiently to overcomethe pull of the magnet whereupon the contacts will be separated quicklyby the energy stored in said convoluted contact blade.

3. A limit switch for use in temperature control systems comprising, incombination, a thermo-responsive actuator, a mounting member, asupporting bracket on said mounting member, a flexible contact bladecarried by said supporting bracket in a position where it may be engagedby said actuator, and at least one contact carried by said mountingmember in a position where it is engagea-ble by said contact blade, amagnet mounted adjacent said contact, and magnetic material associatedwith said contact arm so that the contact arm is attracted toward saidcontact, the contact arm being formed as an S- shaped convolutionbetween the point where it may be engaged by the actuator and the pointWhere it is engageable with the contact, whereby the resiliency of thecontact blade is increased so that energy may be stored therein by theactuator while the magnet is holding the contact blade in engagementwith the contact, until the convoluted contact blade is strainedsufiiciently to overcome the pull of the magnet whereupon the contactswill be separated quickly by the energ stored in said convoluted contactblade.

4. The combination according to claim 1 wherein the contact blade isformed of a first section extending from the supporting bracket to apoint beyond the point where it is engaged by the actuator, it beingthen bent back upon itself to form a second section extending to thevicinity of the supporting bracket and is then again bent back uponitself to form a third section extending beyond the first bend to andbeyond the contact.

5. The combination according to claim 1 wherein the contact blade isformed of a first section extending from the supporting bracket to apoint beyond the point where it is engaged by the actuator, it beingthen bent back upon itself to form a second section extending to thevicinity of the supporting bracket and is then again bent back uponitself to form a third section extending in a direction generallyparallel to the first section and extending beyond the first bend to andbeyond the contact.

6. The combination according to claim 2 wherein the contact blade isformed of a first section extending from the supporting bracket to apoint beyond the point where it is engaged by the actuator, it beingthen bent back upon itself to form a second section extending to thevicinity of the supporting bracket and is then again bent back uponitself to form a third section extending beyond the first bend to andbeyond both contacts.

ALLEN A. DICKE.

REFERENCES CITED The following references are of record in the

